The public switched telephone network (PSTN) is a network of circuit switched equipment and data communication paths that intercoupling circuit switched equipment. The PSTN was initially constructed to service solely voice communications. However, the PSTN was later called upon to service data communications as well. Data communications are now commonly serviced across the PSTN by electronic devices called modems (modulator/demodulator).
The structure and operation of modems is generally well known. Modems couple digital data onto an analog carrier that operates within the frequency bandwidth supported by the PSTN. Current modem technology supports data rates up to approximately 56 Kbps. Modems typically service computers, FAX machines, set top boxes, and other digital devices that require data communication service. In its operation, a modem couples, via the PSTN, to another modem and establishes a communication with the other modem. However, each of these modems services its host device, e.g., host computer, FAX machine, set top box, etc.
As contrasted to packet switched communication networks, e.g., LANs, WANs, Intranets, the Internet, etc., modems service point-to-point circuit-switched communications upon which packet-switched data communications may ride. In one particular modem application, a computer having a modem coupled thereto establishes a communication link across the PSTN with a modem of an Internet Service Provider (ISP). The ISP provides data networking service, e.g., access to the Internet for the computer. The communication link therefore services a data session between the computer and other computers to which the computer may couple via the ISP.
When the modem is not servicing a communication link, it presents an “on-hook” state to the coupled twisted pair of wires that services a local loop to the Central Office (CO) of a servicing telephone network. In the on-hook state, the modem (by a digital access arrangement “DAA Circuit”) circuit of the modem presents high DC impedance across the twisted pair of wires. In the on-hook state, no current flows in the twisted pair of wires from the CO to the modem. In initiating access to the PSTN via the CO, the modem presents an “off-hook” state to the twisted pair of wires of the local loop. In the off-hook state, the modem presents low DC impedance to the CO across the twisted pair of wires and, resultantly, electrical current flows from the CO to the modem. The presence of this electric current flow is an indication to the CO that the telephone line is seized by the modem. Of course, at this time, the CO does not know what type of device has seized the telephone line.
Once the CO detects the off-hook state, the CO provides a dial tone to the modem across the twisted pair of wires. The modem detects this dial tone and determines that the CO is ready to accept number dialing. The modem then dials the number of a desired destination, e.g., telephone number of a modem bank of the ISP, and the CO collects the dialed digits. The CO then processes the call and routes the call to an endpoint corresponding to the telephone number, in this case a terminating modem of the modem band of the ISP. The modems initiate communication and service the data communication. When the data communication is complete, the DAA Circuit of the modem returns the modem to the on-hook state. At this time, the central office detects the lack of current flow to the modem and disables the communication by terminating the call.
Many modems now include modem software that is executed by a processor and a hardware device that is known as a Digital Access Arrangement (DAA) Circuit. The modem software may be executed by a processor resident on a modem card or by a processor resident in the host, e.g., a digital computer's processor. The DAA Circuit interfaces with the modem software via a digital interface and interfaces with the telephone line via an analog interface. With this modem structure, the modem software performs all modulation, demodulation, and control operations.
As is the case with most software, the modem software does not always properly execute and may become nonfunctional for some reason, e.g., modem card processor locks up, host processor locks up, host computer overwrites modem software, etc. When such an event occurs, the modem software becomes nonfunctional with respect to the DAA Circuit. If this happens during modem communication, i.e. when modem is in the off-hook state, the DAA Circuit, not knowing that the modem software is not functioning properly, remains in the off-hook state indefinitely. The telephone line that services the modem is also used for voice communications, FAX communications, set top box communications, and other communications. Thus, when the modem software becomes nonfunctional, the DAA Circuit blocks the telephone line so that the telephone line is unavailable to these other devices.
Heretofore, these operational difficulties have not been solved. Thus, there is a need in the art for a methodology to prevent a modem from blocking the telephone line.